smtplib improvements (#2419)

* Mark SMTP attibutes with class defaults with "= ..."
* Add SMTP.command_encoding
* Annotate SMTP.__exit__()
* Add SMTP.auth() et al.
* Add LMTP_PORT constant

stdlib/3/smtplib.pyi

@@ -1,9 +1,10 @@
 from email.message import Message as _Message
 from socket import socket
 from ssl import SSLContext
+from types import TracebackType
 from typing import (
     Any, AnyStr, Dict, Generic, List, Optional, Sequence, Tuple, Union,
-    Pattern)
+    Pattern, Type, Callable, Protocol, overload)
 import sys
 
 _Reply = Tuple[int, bytes]
@@ -47,24 +48,32 @@ class SMTPAuthenticationError(SMTPResponseException): ...
 def quoteaddr(addrstring: str) -> str: ...
 def quotedata(data: str) -> str: ...
 
+class _AuthObject(Protocol):
+    @overload
+    def __call__(self, challenge: None = ...) -> Optional[str]: ...
+    @overload
+    def __call__(self, challenge: bytes) -> str: ...
+
 class SMTP:
-    debuglevel: int
+    debuglevel: int = ...
     # Type of file should match what socket.makefile() returns
-    file: Any
-    helo_resp: Optional[bytes]
-    ehlo_msg: str
-    ehlo_resp: Optional[bytes]
-    does_esmtp: int
-    default_port: int
+    file: Any = ...
+    helo_resp: Optional[bytes] = ...
+    ehlo_msg: str = ...
+    ehlo_resp: Optional[bytes] = ...
+    does_esmtp: int = ...
+    default_port: int = ...
     timeout: float
     esmtp_features: Dict[str, str]
+    command_encoding: str
     source_address: Optional[_SourceAddress]
     local_hostname: str
     def __init__(self, host: str = ..., port: int = ...,
                  local_hostname: Optional[str] = ..., timeout: float = ...,
                  source_address: Optional[_SourceAddress] = ...) -> None: ...
     def __enter__(self) -> SMTP: ...
-    def __exit__(self, *args) -> None: ...
+    def __exit__(self, exc_type: Optional[Type[Exception]], exc_value: Optional[Exception],
+                 tb: Optional[TracebackType]) -> None: ...
     def set_debuglevel(self, debuglevel: int) -> None: ...
     sock: Optional[socket]
     def connect(self, host: str = ..., port: int = ...,
@@ -87,6 +96,15 @@ class SMTP:
     def expn(self, address: str) -> _Reply: ...
     def ehlo_or_helo_if_needed(self) -> None: ...
     if sys.version_info >= (3, 5):
+        user: str
+        password: str
+        def auth(self, mechanism: str, authobject: _AuthObject, *, initial_response_ok: bool = ...) -> _Reply: ...
+        @overload
+        def auth_cram_md5(self, challenge: None = ...) -> None: ...
+        @overload
+        def auth_cram_md5(self, challenge: bytes) -> str: ...
+        def auth_plain(self, challenge: Optional[bytes] = ...) -> str: ...
+        def auth_login(self, challenge: Optional[bytes] = ...) -> str: ...
         def login(self, user: str, password: str, *,
                   initial_response_ok: bool = ...) -> _Reply: ...
     else:
@@ -114,6 +132,8 @@ class SMTP_SSL(SMTP):
                  source_address: Optional[_SourceAddress] = ...,
                  context: Optional[SSLContext] = ...) -> None: ...
 
+LMTP_PORT: int
+
 class LMTP(SMTP):
     def __init__(self, host: str = ..., port: int = ...,
                  local_hostname: Optional[str] = ...,